Kimberlite Kimberlite Photographed by Michael P. Klimetz Photographed by Michael P. Klimetz "Wyomingite" Photographed by Michael P. Klimetz Green Street Dike Photographed by Michael P. Klimetz Photographed by Michael P. Klimetz Tompkins County Photographed by Michael P. Klimetz Photographed by Michael P. Klimetz Photographed by Michael P. Klimetz Green Street Dike Onondaga County NEW YORK Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.7 g) diamond in 1871 spawned a diamond rush. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today.

The general consensus, reached on kimberlites, is that they are formed deep within the mantle, at depths between 150 and 450 kilometres (93 and 280 mi), from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts. Kimberlite has, in many ways, attracted more attention than its relative volume might suggest that it deserves. This is largely because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth's surface. Furthermore, its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide valuable information on the composition of the deep mantle, and melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle.

Kimberlites occur as carrot-shaped, vertical intrusions termed 'pipes' [see profile below]. This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma which inherits a large proportion of both CO2 and H2O in the system, which produces a deep explosive boiling stage that causes a significant amount of vertical flaring. Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks. The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosive diatreme volcanism from very deep mantle-derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends down to the mantle. Within 1.5–2 km (0.93–1.2 mi) of the surface, the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface.

The surface expression is rarely preserved but is usually similar to a maar volcano. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer (up to 0.6 mile). Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. However, only about 1 in 200 kimberlite pipes contain gem-quality diamonds. The deposits occurring at Kimberley, South Africa were the first recognized and the source of the name. The Kimberley diamonds were originally found in weathered kimberlite which was colored yellow by limonite, and so was called yellow ground.Deeper workings encountered less altered rock, serpentinized kimberlite, which miners call blue ground. Tompkins County Photographed by Michael P. Klimetz Photographed by Michael P. Klimetz Iron Mountain District Photographed by Michael P. Klimetz Murfreesboro Photographed by Michael P. Klimetz Murfreesboro Photographed by Michael P. Klimetz [Core Section] Kimberlite Kimberlite Kimberlite is not a specific mineral.

It is rather a type of potassium-rich volcanic rock best known for sometimes containing diamonds . "It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.7 g) diamond in 1871 spawned a diamond rush, eventually creating the Big Hole. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today." (Wikipedia) This sample of kimberlite is displayed in the Smithsonian Museum of Natural History. This sample of kimberlite is about 25 cm across and is from Murfreesboro, Arkansas. Volcanic Minerals Volcanic Minerals Can your knowledge of volcanic rocks make you rich? Yes!! That is, if you combine it with knowledge of mineral deposits and global economic factors. Volcanoes directly or indirectly produce or host deposits of aluminum, diamonds, gold, nickel, lead, zinc, and copper. We use most of these materials everyday and, over the course of a lifetime, consume some of them (via the products we buy and use) in great amounts. This page provides a overview of the mineral deposits hosted in volcanic rocks. .

Photo courtesy of Grant Boxer. Bauxite Aluminum ore, called bauxite, is most commonly formed in deeply weathered rocks. In some locations, deeply weather volcanic rocks, usually , form bauxite deposits. This sample of bauxite ore is from Western Australia. Uses for Aluminum Aluminum has a wide range of common uses. It is lightweight, strong (especially with alloys), and conducts heat well. Many kitchen items (pots and pans, foil, dishes) are made of aluminum. Most materials used for transportation use large amounts of aluminum: cars, trucks, boats, aircraft, and aircraft engines. Road signs and high-voltage power lines are also made mostly of aluminum. World Supply of Aluminum About 110 million tonnes of aluminum was produced in 1994. Australia produces most of the world's aluminum. Diagram from ITAM Bauxite by the Minerals Council of Australia. Nickel Deposits of nickel sulfides are mined from greenstone belts in many ancient volcanic terranes. The ore is associated with . Komatiites have a unique texture called spinifex. The texture is an intergrowth of long, skeletal crystals of olivine and/or pyroxene.

The texture is named for its resemblance to spinifex grass in Australia. Photo courtesy of Rob Hill, CSIRO. Simplified from Hill and others, 1989. Cross-section of igneous lithologies and associated massive nickel sulfides in a komatiite lava flow. Simplified from Hill and others, 1989. Core in komatiite lava with adcumulate texture. Photo courtesy of Rob Hill, CSIRO. Uses for Nickel: Nickel is commonly alloyed with other metals to provide resistance to corrosion and heat, and to add strength and hardness . These alloys are used for industrial and consumer goods. Most of the nickel produced is used to make stainless steel. Stainless steel contains about 8% nickel. Monel metal, a highly corrosion-resistant alloy, contains 67% nickel and is usedin ship building, food-processing equipment and in hospitals. Nickel is used by the military for armor plate. Nickel is used in alkaline batteries, dyes, insecticides and as a catalyst. Nickel is commonly used in coins. World Supply of Nickel The Sudbury District in Canada is the world's largest producer of nickel.

Another major nickel producer is in Western Australia and includes the Kambalda and MurrinMurrin deposits. MurrinMurrin consists of five deposits with a total of 11.9 million tonnes at 1.01% Ni and 0.07% Co. In 1993, a large deposit of nickel-, copper-, and cobalt-sulfides (~150 million tons) was discovered at Voisey Bay on the northeast coast of Labrador, Canada. This deposit formed in magma chambers that fed large basaltic eruptions. In 1994, about 875,000 tonnes of nickel was produced worldwide. Major producers are the Commonwealth of Independent States (former Soviet Union), Canada, New Caledonia, Indonesia, and Australia. Diagram from ITAM Nickel by the Minerals Council of Australia. Gold of Vesuvius. Gold forms in close association with volcanoes or is hosted in volcanic rocks. Three environments/styles are most common: gold in greenstone belts, gold in porphyry deposits, and gold in epithermal deposits.

Gold in Greenstone Belts Gold is found in Archean (rocks older than 2.5 billion years) greenstone belts in Australia, southern Africa, and Canada. Greenstone belts are volcanic-sedimentary sequences, which include shearing . Gold is most commonly found along the edges of greenstone belts and associated with structural features. Intensely altered and fractured basalt is a common host rock. The gold is though to be mobilized by hydrothermal solutions during regional metamorphism. The solutions probably contain only a few parts-per billion gold but great volumes of solution can precipitate their gold in a small zone with favorable chemical conditions. The deposit itself is usually a quartz vein that carries the gold or adjacent altered rock. A classic example of gold hosted in greenstone is the Golden Mile in Kalgoorlie, Western Australia. The top diagram is a simple map that shows folds, faults, rock type, and mineralization. The bottom diagram is a cross-section of the deposit. Dolerite refers to rocks with plagioclase crystals in pyroxene crystals. By 1993, 40 million ounces of gold has been mined from the Golden Mile. Diagram from ITAM Gold by the Minerals Council of Australia.

Another example of gold hosted in greenstone is the St. Ives deposit near Kambalda, Western Australia. Three mines have produced more than 2.1 million ounces of gold from 1980-1993. Another 5.4 million ounces of gold remains in the deposits. The gold is found in altered rocks in all parts of the sequence. Diagram from ITAM Gold by the Minerals Council of Australia. Gold and copper are found in ore bodies associated with porphry. Porphry is a general term applied to igneous rocks of any composition that contain conspicuous phenocrysts (crystals) in a fine-grained groundmass. The term is from a Greek word for purple dye and was first applied to a purple-red rock with phenocrysts of alkali feldspar that was quarried in Egypt. Diagram from ITAM Copper by the Minerals Council of Australia. This type of deposit forms beneath stratovolcanoes and is associated with subduction zones . Erosion strips off overlying rocks to expose the mineralization. Gold and copper are found in sulfide minerals disseminated throughout the large volumes of intrusive rock (strictly speaking, this ore is associated with volcanic systems, usually not the volcanoes themselves).

This requires large amounts of rock to be mined, often in open pits. The deposits are commonly 3-8 km across and copper may be less than 1% of the rock. Porphyry deposits are zoned in alteration (potassic ® sericitic ® argillic ® propylitic) and mineralization. Epithermal refers to mineral deposits that form in association with hot waters. The deposits form within 1 km of the surface and water temperatures are about 50-200 degrees C. Shallow bodies of magma supply heat. The rising hot water carries dissolved gold and other elements. The water boils about 300 m below the surface and hydrogen sulfide gas escapes. This causes the gold to precipitate. The boiling zone is the target for mineral exploration. Veins commonly host the economic minerals. Diagram from ITAM Gold by the Minerals Council of Australia. Lihir Island in Papua New Guinea Lihir Island is an epithermal deposit discovered in 1982. The island is made of three volcanoes including Luise trachybasalt lava flows, breccia, and tuffs.The mineralized rocks are highly altered.

Most of the ore is in breccia thought to have been a boiling zone for rising fluids. The deposit formed between 350,000 and 100,000 years ago. It is estimated that the deposit contains 21.3 million ounces proven and probably another 42 million ounces as a geological resource. Most of the gold is fine particles in pyrite (FeS) grains. Hot springs and fumaroles are still active on the caldera floor. Uses for Gold Gold is rare, durable, chemically inert, and beautiful. These qualities make it useful for monetary exchange and investment, jewelry, and art.Gold’s high electrical conductivity, malleability, and ductility make it useful as an industrial metal. Gold is alloyed with silver, copper, nickel, palladium, zinc, and titanium to increase its tensile strength. Note: for gold, 1 carat = 1/24 part. Pure gold is 24 carat. World Supply of Gold Below is a pie diagram of major gold producers. In 1994, 2,296 tonnes of gold was produced.

South Africa is the largest producer, followed by the USA, Australia, and the CIS (former Soviet Union). Diagram from ITAM Gold by the Minerals Council of Australia. Diamonds are crystalline carbon and the hardest known substance. This photo shows diamonds from the Argyle Mine in northern Western Australia. xenoliths are found in some kimberlite and provide clues to the magma's origin. The source depth for kimberlite magmas is estimated at 200 km, more than twice as deep as the source region for most magmas. At a depth of 200 km the pressure is 60,000 times greater than the surface and the temperature is about 1500 C. Kimberlite magmas are rich in carbon dioxide and water which brings the magma quickly and violently to the surface. Most kimberlites occur as multiple intrusive events. Kimberlite was named for the rock associated with diamonds in Kimberley, South Africa. Right: Simplified from Hawthorne (1975). Kimberlite magmas form "pipes" as they erupt. A deposits. In the subsurface, a funnel-shaped body narrows to a depth of hundreds of meters.

The pipe (also called a diatreme) is filled with kimberlite, with or without diamonds (only 1 in 5 of the pipes at Kimberley contain diamonds). Just how many diamonds are needed to make aa pipe economical? Some South African mines operate at 25 carats of diamond per 100 cubic meters of rock or about 2 grams of diamonds per 100 tons of rock. Because diamond has a specific gravity of 3.5 grams per cubic centimeter, 1 cubic centimeter of diamond weighs 16 carats. Picture a giant 100-ton ore truck full of kimberlite - that truck contains only half of a cubic centimeter of diamonds! Only about 35% of those diamonds are gem quality. Macles diamonds from the Argyle pipe, Western Australia. Photo courtesy Grand Boxer. Uses for Diamonds Most diamonds are used in drill bits and diamond tools. A small number are used for glass cutters and surgical instruments. Only the finest are used as gems. World Supply of Diamonds Australia is currently the world's largest producer of diamonds. Most of these diamonds are low quality and used for industrial purposes.

Most of the diamonds are from the Argyle diamond pipe in northern Western Australia. The pipe at Argyle is made of lamproite, not kimberlite. The mine produced 27.8 million carats (1 ct = 200 mg; 5 carats = 1 gram) of low grade diamonds in 1993-1994. Because of the high rate of production at Argyle, mining operations will end within the next few years. The mines at Kimberley, South Africa have produced a total of more than 200 million carats since the 1870s. About half of South Africa’s diamonds are gem quality. Base Metals Zinc-Copper volcanogenic massive sulfide (VMS) deposits have been observed as they form at mid-ocean ridges. Chimneys formed at the ridges have as much as 29 weight % zinc and 6 weight % copper in sulfide minerals (pyrrhotite, pyrite, sphalerite, and chalcopyrite). The minerals are dissolved in fluids at temperatures as high as 380 C flowing at 1-5 m/sec.

The minerals precipitate as the hot solution comes in contact with cold sea water. Photography courtesy of Woods Hole Oceanographic Institution and members of the Adventure dive (Principle Investigators: D. Fornari, R. Haymon, K. Von Damm, M. Perfit, M. Lilley, and R. Lutz). Lead, zinc, and cooper are found in VMS deposits. The deposits form in deep ocean water by the precipitation of sulfide minerals released by submarine volcanoes. Volcanic fluids and hot seawater move through the volcanic rocks and leach metals. The deposits are associated with lava flows, breccia, water-deposited tuffs, cherts, sulfates, and limestones. VMS deposits are usually associated with quartz, anhydrite, gypsum, and barite. This photo shows pyrite in silicified tuff from a VMS deposit in eastern Java, Indonesia. The sample contains 0.55 ppm gold. There are three types of VMS deposits: ga, sp, py, some ch ch, py volcanic arc ) basins.

Kuroko deposits are zoned from copper-rich near the center, to zinc-rich, to lead-rich at the outer edges of the deposit. The fluids that form Kuroko deposits have twice the salinity of average ocean water and temperatures of 250-300 C. This cross-section of a typical Kuroko deposit is from Sato (1974) and Franklin and others (1981). Modern-day VMS deposits of the Kuroko-style have been observed as they form in the back-arc basin of the Okinawa Trough (Halbach and others, 1989). The deposit at Bathurst in Canada is an example of lead-zinc-copper VMS. Hydrothermal solutions associated with Keweenawan flood basalts have produced copper deposits in the Lake Superior area. Between 1845 and 1968, over 13 billion pounds of copper and 16 million ounces of silver were produced from the Keweenawan district. The porphry (not VMS) mine at El Abra in Chile will produce 500 million pounds of copper annually.

Uses for Copper, Lead, and Zinc Copper was used as long ago as 8000 BC for tools, weapons, and ornaments. The discovery of bronze (copper and tin alloy) about 3500 BC marked the onset of the Bronze Age. Romans used lead for the plumbing and sewage systems. In Modern times, the electrical conductivity, ductility, and resistance to corrosion of copper, lead, and zinc, make them very useful in alloys. Copper is used for electrical applications (50%), general and industrial engineering applications (20%), building and construction (15%), transportation (11%) and other applications. Lead is used in storage batteries, paints, dyes, explosives, insecticides, and rubber products. Zinc is used in galvanized steel, protective coatings for steel, and die casting. Zinc compounds are used for luminous dials, cosmetics, plastics, rubber products, soaps, and inks. World Supply of Copper In 1991, 9,167,000 tons of copper was produced. Chile (19.8%), USA (17.8 %), and the CIS (9.8%) are the main producers.

USA, Japan, and the CIS are the main consumers. Obsidian By Jim Miller, B.Sc., M.Sc. Geology* Obsidian...even the name is exotic. Ever since I had my first rock collection as a child, I've loved obsidian. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. That amazing trip really opened my eyes. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone. matrix . When I say "crystals," don't visualize those beautiful pointed prisms of quartz found in geodes. All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral.

Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces. Granite is composed entirely of intergrown crystals of quartz, feldspar, mica and other minerals. These relatively large mineral crystals (easily visible to the naked eye) give granite a rough surface. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. The intersections of conchoidal fracture surfaces can be sharper than a razor. This had obvious advantages for our Stone Age ancestors, who used obsidian extensively for tool making. rhyolite , which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a congealed liquid with minor amounts of microscopic mineral crystals and rock impurities. Obsidian is relatively soft with a typical of 5 to 5.5 on the mineral hardness scale. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0.

Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the of obsidian. Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon. If obsidian is similar in composition to granite and rhyolite, both of which were originally molten, then why is obsidian glassy? The answer relates to the original cooling rate and water content of the magma. Granite cools very slowly miles below the surface of the earth; this slow cooling over millions of years allows for the formation of sizable mineral crystals within the slowly cooling mass of molten rock. Rhyolite typically cools more rapidly near the earth's surface and contains smaller mineral crystals than granite. When rhyolite magma approaches the earth's surface and the pressure of burial decreases, most of the water in the magma is lost as steam.

The resulting silica-rich magma with little remaining water becomes very viscous (thick and pasty) obsidian magma. This magma is so viscous that sizable mineral crystals cannot grow before chilling of the magma "freezes" crystal development. Some obsidian is erupted as lava flows at the ground surface. These surface flows are so viscous that they flow very slowly. One article I read indicated that "an ant could probably outrun an obsidian , a glassy, bubble-rich, lightweight rock that develops when water vapor (steam) escapes rapidly from the molten glass at or near the ground surface. Sometimes obsidian of excellent quality develops as surface lava flows. However, the best quality obsidian often forms below the ground surface around volcanic vents. Silica-rich magma squeezes into rock fractures to form layers and lenses of obsidian that are relatively free of dirt, viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colors. Each of these streaks or lines may represent a distinct pulse of an obsidian eruption.

You can visualize the process that results in streaked obsidian if you consider two blobs of green and red taffy (a viscous candy) that are mixed together. Distinct streaks of red and green taffy result as the blobs are mixed. In the case of obsidian, the slow flow of stiff, viscous magma away from the source vent provides the mixing needed to create the layered or streaked varieties of obsidian. The "midnight lace" variety of obsidian often has incredibly contorted streaking, apparently formed as the obsidian layers were stretched and rolled with slow movement of the magma. The various colors of obsidian are a result of several factors. Clear varieties of obsidian contain very few opaque impurities or microscopic mineral crystals. Red or brown obsidian generally results from tiny crystals or inclusions of hematite or limonite (iron oxide). Abundant, microscopic crystals of minerals like magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny fragments of rock, likely produce the jet-black varieties of obsidian. Microscopic crystals of various types of feldspars may yield the unique blue, green, purple or bronze colors associated with rainbow obsidian.

The reflectance of rainbow obsidian is likely attributed to a preferred orientation of microscopic crystals of feldspar or mica oriented along flow layers. A certain amount of water always is present in obsidian. Very small inclusions of water vapor in the form of bubbles often are trapped in the glass. Tiny gas bubbles that have been stretched nearly flat along the flow layers in obsidian generally cause the reflectance of gold sheen and silver sheen obsidian. Some of these bubbles are visible to the naked eye. The bubbles can be seen readily with a strong magnifying glass or a microscope. Obsidian is relatively unstable from a geologic perspective. It is rare to find obsidian older than about 20 million years, which is very youthful in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock in a process known as "devitrification." In this process, the silica molecules within the glass slowly rearrange into organized crystal patterns. The "snowflakes" in snowflake obsidian are quartz crystals that have formed through devitrification of the original obsidian. The crystals that develop through devitrification cause obsidian to lose its conchoidal fracture and glassy texture.

Native Americans discovered almost all of the obsidian locations in North America. Each obsidian source area has a unique assemblage of trace elements, allowing identification of the original source locality for the obsidian used in prehistoric artifacts. Trace element analyses have shown that native peoples traded this valuable commodity many hundreds of miles from the volcanic source areas along numerous trade routes. The fact that obsidian was transported great distances attests to the mystique of this unique material. My favorite place to collect obsidian is Glass Buttes in central Oregon. A trip to Glass Buttes is well worth the effort for the natural scenery, abundant and beautiful gem-quality obsidian, and exploration of a fascinating geological area. These sagebrush- and juniper-covered hills are remote but accessible by automobile. Located in central Oregon south of Highway 20 and about 80 miles east of Bend, a visitor will be rewarded with many different varieties of obsidian through exploration of the area. Fist-sized pieces are abundant and can be collected with no digging. For large pieces of high quality obsidian (ranging up to several hundred pounds), considerable effort with a shovel and pry bar are necessary.

Further information on Glass Buttes can be found in several rockhound guidebooks that include the central Oregon area. * Jim Miller is a practicing geologist and an avid flint knapper residing in Bothell, Washington. Examples of obsidian from Glass Buttes and stone tools made from those materials by the author are included with this article. Several varieties of obsidian from Glass Buttes, Oregon and tools made by the author from those materials. From left to right across the top, the obsidian varieties are silver sheen, midnight lace, pumpkin and red mahogany. Source material and stone tools of red mahogany obsidian. Source material and stone tools of red mahogany obsidian. Pencil gives scale. Source material and stone tool of silver sheen obsidian. Source material and stone tools of pumpkin obsidian. Pencil gives scale. Multi-colored streaks oriented along flow lines in midnight lace obsidian. Contorted streaks within midnight lace obsidian. viscosity that has the greatest control on the ordered crystalline pattern, the measure of viscosity is dependent on the temperature, crystal content and chemical composition .

Viscosity is a measure on the ability of substance to flow, high viscosity means poor ability to flow and low viscosity means good ability to flow, an example of rhyolite . For obsidian to form, magma is trapped below the eutectic, point of crystallization, by loss of heat. Therefore leaving a magma that is unable to crystallize will form (glass) obsidian. For this process to occur during a lava flow the lava is caught just below crystallization temperature, thus forming a glass due to the inability to form a crystalline solid. The formation of obsidian could also be the melt, liquid remaining from a magma after crystallization, of a rhyolite magma that has been erupted before any crystals can form as stated earlier. The gas content of obsidian is very low so for this to occur the gas has to be released in some way before the dacite . This difference is due to the difference in composition, specifically volatile, gas, content. Volatiles within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily so as they rupture, burst, they release an enormous amount of pressure producing an eruption such as the Mount. St.

Helens eruption on May 18th 1980. For these explosive types of eruptions not occur in the emplacement of obsidian the volatile content for the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977). The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the may property controling the viscosity over any of the other properties. The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference.

Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian. As volatile as pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C is than released as the obsidian at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma.

This information comes to the conclusion that the flow of obsidian is dependent on the time for the obsidian magma melt to cool, and that amount of time for cooling is very short because if allowed to cool to a temperature lower than initial temp it will not flow. Bibliography: Bakken, B., (1977), Obsidian and Its Formation. North West Geology. 6-2, (88-92) Obsidian and its Groovy Flow Emplacement A model for rhyolite composition approaches the surface volatiles exsolve, separate, from the magma and rise to the plumes top. The decrease in the confining pressure leads to frothing of the volatile rich top section of the plume as ascension of the stratified mass occurs. This process of vesiculation leads to some number of explosive eruptions depositing . The transition from the explosive eruption to the pumiceous lava occurs once gas pressure in vesicles can no longer exceed strength of magma.

Later in the eruption following the pumiceous lava flow the event of the emplacement of obsidian occurs, this process happens with the bubble free obsidian flowing out over the earlier emplaced coarsely vesicular pumice (highly inflated pumiceous lava). The Obsidian is slightly less viscous but more dense than the coarse pumice. There remains a gradual transition from the coarse pumice to obisidian due to volatile decrease with the progression from pumice phase to obsidian phase. Essentially while obsidian is erupted vesicular material continues to be extruded. As the obsidian rides over the coarse pumice, volatiles continue to evolve from the cooling upper flow surface of the obsidian flow. This process of the volatiles exsolving from the obsidian flow forms a finely vesicular layer that keeps the flow interior of the obsidian insulated and warm; this contact is also gradational, just as the coarse pumice to the obsidian was.

Within the volcanic vent that the obsidian flowed from the remaining magma from within cools and crystallizes and then erupts as a crystal rich rhyolite flow capping the vent mouth as a rhyolite pyroclastic flow occurred essentially taking the place of the pumiceous lava flow, however there remain deposits of course pumice within the region of Big Obsidian Flow which many indicate a short event of pumiceous flow. The final stage was the emplacement of Big Obsidian Flow that moved very slowly and according to Sherrod et al (1997) this lava flow probably moved a few meters to tens of meters a day. From observations in the field of the Big Obsidian Flow the presence of a thin visculated layer coats some of the surface of the obsidian flow. The final stage of the emplacement of a rhyolite dome is not present the vent is actually plugged by the obsidian flow indicating that the rhyolite stage was not present in the eruption or possibly the rhyolite magma was already to cool and harden to come to the surface of the flow.

This is an image of the Big Obsidian Flow of Newberry Volcano,a USGS Photograph taken in October 1987 by Willie Scott. The structure of the deposit indicates that the obsidian flowed over being erupted explosively. Fink (1983) provided most of the information regarding the flow emplacement model of obsidian as well as the figures displayed. Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite . This difference is due to the difference in composition, specifically gas content. Gases within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily from the magma so as they rupture they release an enormous amount of pressure producing an eruption such as the Mount. St. Helens eruption on May 18th 1980. For these explosive types of eruptions not occur in the emplacement of obsidian the gas content of the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977).

The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the property controling the viscosity over any of the other properties, higher the temperature the lower the viscosity. The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference. Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian.

As volatile rich pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C the obsidian melt can be erupted at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma. This information comes to the conclusion that the flow of obsidian is dependent on the time the obsidian is erupted during the eruption, the temperature of the obsidian and the gas content. Bibliography: Fink, J.H., (1983), Structure and emplacement of a Rhyolite Obsidian Flow: Little Glass Mountain, Medicine Lake Highland, Northern California.

Geological Society of America Bulletin. 94 (362-380) Sherrod., Mastin., Scott., Schilling. (1997). Newberry , Oregon Big Obsidian Flow - Latest Eruptive Event Excerpt from: Sherrod, Mastin, Scott, and Schilling, 1997, Volcano Hazards at Newberry Volcano, Oregon. U.S. Geological Survey Open-File Report, 97(513) The Fly Style of Obsidian Flow Textures and Features Obsidian flows usually consist of fold surfaces, explosion craters, flow banding, and cavities underling the craters. The explosion craters are commonly located on the surface of the flow and can have diameters of 10-25m (36-91ft), at 5-15m (15-55ft) deep; the accompanying cavities can be 5-15m in length. The cavities form near the surface by a buckling mechanism.

Buckling occurs by contrast of stiff upper layer relative to the near liquid flow interior, which forces the upper layer to make an almost triangular shape at the surface, separating it from the lower layer. These cavities can collect /meteoric water*, and as vapor pressure builds up and exceeds the strength of the surface crust of cavity an explosion results producing the explosion caters. *Meteoric water: rain water/ground water, not from the magma . Image depics the event of cavities forming by buckling and exploding due to gas pressure exceeding the strength of the rock. This illustration comes from Castro's Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano . Flow banding in obsidian is a distinctive characteristic and the abundance of the flow banding. Even with the abundance of these flow bands within obsidian little is known about the origins of these features, but what is known is that these freatures derive from both crystallization and deformation processes. According to a paper by J.M. Castro the flow bands develope from the deformation of microlites (very small ) by shear strains during the flow of the obsidian. The picture below shows the flow banding as pointed out by the arrow.

Bibliography Castro, J., Cashman, K., Jaslin, N., Olmsted, B., (2002) Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano. Journal of Volcanology and Geothermal Research. 114 (3-4), (313-330) Castro, J. (2005). New insignts on the origin of flow bands in obsidian. Special Paper - Geological Society of America, 39655-65. doi:10.1130/2005.2396(05). Water Plus Obsidian Obsidian After an obsidian flow has been emplaced it is subject to the atmosphere that causes weathering to the obsidian. One specific type of weathering done on to obsidian is called hydration, which occurs by the water within the atmosphere being absorbed by the obsidian thus increasing the water content within the rock. The product of a hydrated obsidian is called devitrified obsidian, (de- to remove) (vitrify- glassy), so essentially the removal of the glassy property of the obsidian.

The products of devitrified obsidian can produce secondary fiberous mineral crystals that can form ball like shapes called spherulites that are embedded within the obsidian, these secondary minerlas can develope into what is called snowflake obsidian as well, which makes the obsidian look as if it is decorated with snowflackes due to the snowflake shapes the secondary minerals produce. The snowflake obsidian can be used as jewlery and many rockhounds search for it, Yellowstone This rock is fine-crystalline, non-equigranular to porphyritic and consists of olivine, serpentine, garnet, ilmenite, pyroxene, calcite and magnetite. Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5 carat diamond in 1871 spawned a diamond rush, eventually creating the Big Hole. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today.

The general consensus reached on kimberlites is that they are formed deep within the mantle, at between 150 and 450 kilometres depth, from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts. Kimberlite K Kimberlite Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds . It is named after the town of Kimberley in South Africa , where the discovery of an 83.5 carats diamond in 1871 spawned a diamond rush, eventually creating the Big Hole. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today. The general consensus reached on kimberlites is that they are formed deep within the mantle, at between 150 and 450 kilometres depth, from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components.

It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts. Kimberlite has in many ways attracted more attention than its relative volume might suggest that it deserves. This is largely because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth's surface. Furthermore, its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide valuable information on the composition of the deep mantle, and melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle. Morphology and volcanology Kimberlites occur as carrot shaped, vertical intrusions termed 'pipe'. This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma that inherit a large proportion of both CO2 and H2O in the system which produces a deep explosive boiling stage that causes a significant amount of vertical flaring (Bergman, 1987).

Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks (Clement and Skinner 1985, and Clement, 1982). The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosive diatreme volcanism from very deep mantle derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends down to the mantle. Within 1.5-2 km of the surface the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved but is usually similar to a maar volcano. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer. Two Jurassic kimberlite dikes exist in Pennsylvania .

One, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered by miners. Petrology Both the location and origin of kimberlitic magmas are areas of contention. Their extreme enrichment and geochemistry has led to a large amount of speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even as deep as the transition zone. The mechanism of enrichment has also been the topic of interest with models including partial melting, assimilation of subducted sediment or derivation from a primary magma source. Historically, kimberlites have been subdivided into two distinct varieties termed 'basaltic' and 'micaceous' based primarily on petrographic observations (Wagner, 1914). This was later revised by Smith (1983) who re-named these divisions Group I and Group II based on the isotopic affinities of these rocks using the Nd, Sr and Pb systems.

Mitchell (1995) later proposed that these group I and II kimberlites display such distinct differences, that they may not be as closely related as once thought. He showed that Group II kimberlites actually show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion. Group I kimberlites Group I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromian pyrope, almandine-pyrope, chromian diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite.

Group I kimberlites exhibit a distinctive inequigranular texture cause by macrocrystic (0.5-10 mm) to megacrystic (10-200 mm) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite in a fine to medium grained groundmass. The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic olivine , pyrope garnet, Cr-diopside, magnesian ilmenite and spinel . Group II kimberlites Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of orangeites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.

Characteristic primary phases in the groundmass include: zoned pyroxenes (cores of diopside rimmed by Ti-aegirine); spinel-group minerals (magnesian chromite to titaniferous magnetite ); Sr- and REE-rich perovskite; Sr-rich apatite ; REE-rich phosphates ( monazite , daqingshanite); potassian barian hollandite group minerals; Nb-bearing rutile and Mn-bearing ilmenite. Kimberlitic indicator minerals Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with peculiar chemical compositions. These minerals such as potassic richterite, chromian diopside (a pyroxene ), chromium spinels, magnesian ilmenite, and garnets rich in pyrope plus chromium are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites. These indicator minerals are generally sought in stream sediments in modern alluvial material . Their presence, when found, may be indicative of the presence of a kimberlite within the erosional watershed which has produced the alluvium.

Geochemistry The geochemistry of Kimberlites is defined by the following parameters; Ultramafic; MgO >12% and generally >15% Ultrapotassic; Molar K2O/Al2O3 >3 Near-primitive Ni (>400 ppm), Cr (>1000 ppm), Co (>150 ppm) REE-enrichment Moderate to high LILE enrichment; ΣLILE = >1,000 ppm High H2O and CO2 Economic importance Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. However, only about 1 in 200 kimberlite pipes contain gem-quality diamonds. The deposits occurring at Kimberley, South Africa were the first recognized and the source of the name. The Kimberley diamonds were originally found in weathered kimberlite which was colored yellow by limonite , and so was called yellow ground. Deeper workings encountered less altered rock, serpentinized kimberlite, which miners call blue ground. See also Udachnaya pipe. The blue and yellow ground were both prolific producers of diamonds.

After the yellow ground had been exhausted, miners in the late 1800's accidentally cut into the blue ground and found gem quality diamonds in quantity. The economic importance of the time is that with flood of diamonds being found, the miners were undercutting each other's price of the diamonds and eventually decreased the diamonds value down to cost in a short time. Diamonds | Volcano World | Oregon State University OREGON STATE UNIVERSITY Diamonds are crystalline carbon and the hardest known substance. This photo shows diamonds from the Argyle Mine in northern Western Australia. xenoliths are found in some kimberlite and provide clues to the magma's origin. The source depth for kimberlite magmas is estimated at 200 km, more than twice as deep as the source region for most magmas. At a depth of 200 km the pressure is 60,000 times greater than the surface and the temperature is about 1500 C. Kimberlite magmas are rich in carbon dioxide and water which brings the magma quickly and violently to the surface. Most kimberlites occur as multiple intrusive events. Kimberlite was named for the rock associated with diamonds in Kimberley, South Africa.

Right: Simplified from Hawthorne (1975). Kimberlite magmas form "pipes" as they erupt. A deposits. In the subsurface, a funnel-shaped body narrows to a depth of hundreds of meters. The pipe (also called a diatreme) is filled with kimberlite, with or without diamonds (only 1 in 5 of the pipes at Kimberley contain diamonds). Just how many diamonds are needed to make aa pipe economical? Some South African mines operate at 25 carats of diamond per 100 cubic meters of rock or about 2 grams of diamonds per 100 tons of rock. Because diamond has a specific gravity of 3.5 grams per cubic centimeter, 1 cubic centimeter of diamond weighs 16 carats. Picture a giant 100-ton ore truck full of kimberlite - that truck contains only half of a cubic centimeter of diamonds! Only about 35% of those diamonds are gem quality. Macles diamonds from the Argyle pipe, Western Australia. Photo courtesy Grand Boxer. Uses for Diamonds Most diamonds are used in drill bits and diamond tools. A small number are used for glass cutters and surgical instruments.

Only the finest are used as gems. World Supply of Diamonds Australia is currently the world's largest producer of diamonds. Most of these diamonds are low quality and used for industrial purposes. Most of the diamonds are from the Argyle diamond pipe in northern Western Australia. The pipe at Argyle is made of lamproite, not kimberlite. The mine produced 27.8 million carats (1 ct = 200 mg; 5 carats = 1 gram) of low grade diamonds in 1993-1994. Because of the high rate of production at Argyle, mining operations will end within the next few years. The mines at Kimberley, South Africa have produced a total of more than 200 million carats since the 1870s. About half of South Africa’s diamonds are gem quality. KIMBERLITE - The source of diamonds KIMBERLITE - The source of diamonds KIMBERLITE: the source of diamonds Diamond-bearing ‘blue ground’ from Kimberley, South Africa Kimberlite is a rare type of volcanic rock best known for sometimes containing diamonds.

It is named after the town of Kimberley in South Africa, where the rock was first recognised, and the most famous diamond-producing area in the world. Provided in a plastic display box with a comprehensive label as shown. WHAT IS KIMBERLITE? Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes which are the most important source of mined diamonds today. Kimberlites were formed deep within the Earth’s mantle – it is thought that kimberlite magma is produced at a depth of more than 150 kilometres - and were erupted at the surface rapidly and violently. In addition to carrying diamonds to the surface, kimberlite is our only source of samples of some types of rocks that were formed at those depths, therefore providing information about the Earth’s interior. The Kimberley diamonds were originally found in weathered kimberlite which was coloured yellow by iron oxide, and so was called ‘yellow ground’. Deeper workings encountered less altered rock, which miners called ‘blue ground’. Please note: We have a very limited number of pieces of kimberlite in stock. The photographs are typical examples of what we have. You will receive the best example from our current stock.

Click on a picture for a larger image Size: 6 x 4 x 3.5 centimetres (size of box) Weight: varies from 20 to 30 grams Price per piece Prospector's Guide to Diamonds - Kimberlite Prospector's Guide to Diamonds Diamonds are a Geologist's Best Friend More KIMBERLITE Kimberlite is just one of several rock types found at the earth's surface that sometimes contains diamonds. More than 40 kimberlites were initially mapped in the Colorado-Wyoming State Line district (1), this was followed by mapping of a small kimberlite blow in the Sheep Rock district (2), a major dike-blow complex in the Iron Mountain district (3), and the mapping of a major lamproite district in the Leucite Hills field (4). During research of this region, hundreds of other anomalies were found. Highwall exposing (blue-ground) kimberlite breccia at the Kelsey Lake diamond mine, Colorado. The kimberlites occur as pipes, dikes and blows. Most appear to be deeply eroded suggsting a possibility for significant diamond placers downstream.

Even so, there has been absolutely no exploration for diamond placers in this region although one Ft. Collins prospector recovered diamonds in the early 1970s while looking for gold in Rabbit Creek, another panned a diamond with little effort in George Creek, and placer diamonds were recovered in Fish Creek during exploration of Kelsey Lake (including a 6.2 carat diamond). Cross section of Colorado-Wyoming kimberlite. Kimberlites found to date are primarily diatreme facies with lesser hypabyssal facies. Many were initially found based on blue ground exposures, vegetation anomalies, KIMs, mantle nodues, an d depressions. Others were found by geophysics which led to the discovery of another group of kimberlite in the State Line district that were found with IMPUT geophysics, but all remain unexplored and none drilled! Left to right - blue ground at Aultman kimberlite, Wyoming exposed by badger. Drilling at Aultman kimberlite intersects kimberlite at shallow depth (dark gray soil) and granite (reddish soil).

Contact exposed at Schaffer kimberlite showing the knife sharp contact between the blue ground kimberlite and the sheared granite. Diamond grades of kimberlites range from a trace diamond to 135 carats/100 tonnes based on diamond mills in the district that were inefficient and typically rejected diamonds of all sizes. Thus actual grades are greater. The size of recovered diamonds ranged from micros to the largest diamond - 28.3 carat gemstone. One fragment from a broken octahedron orignated from a diamond of at least 80 to 90 carats. Much larger diamonds may have been rejected by mills - for instance, the mill at Kelsey Lake Colorado was poorly designed and rejected any diamond >40 carats in weight! It also rejected smaller diamonds. Bernie Free poses next to large granite xenolith in mine rib at the Sloan 2 kimberlite, Colorado. During recent exploration of the region by the author, several hundred cryptovolcanic structures were identified within the State Line district as well as to the south and north.

Most are likely kimberlites based on the distinct presence of blue ground, circular to elliptical depressions that are structually controlled, and most enclosing large, rounded boulders. Potentially, 10 to 12 previously unknown kimberlite districts were discovered along with the numerous kimberlites overlooked within the State Line district. It is likely that the Colorado-Wyoming region is underlain by one of the largest kimberlite-lamproite-lamprophyre provinces in the world! A few kimberlites in the district had drilled resources. These include the Kelsey Lake group and the Sloan Ranch group of kimberlites. Because of the large number of kimberlites in this region that are diamondiferous and are deeply eroded, it is estimated that as much as 2,000 feet of the vertical column of the pipes have eroded and their diamond content carried downstream. Thus it is highly likely that streams like Fish Creek, Rabbit Creek, George Creek and many others draining into the Poudre River in Colorado contain millions of diamonds from large to small - some potentially worth several thousand to more than a hundred thousand dollars based on diamonds sold from Kelsey Lake.

Diamonds are the hardest natural substance on earth and in southern Africa, have been shown to transport as far as 600 miles from their known source kimberlites in the Kimberley region to the ocean shores along western Africa. Prospecting streams for diamonds in this region of Colorado for many miles downstream could be very productive. Left - Hypabyssal facies diamondiferous kimberlite at the Sloan Ranch, Colorado. Note the large pyrope megacryst (gem-quality Cape Ruby). Below - Hypabyssal facies diamondiferous kimberlite with large chromian diopside (Cape Emerald) in matrix. Create your own free website today Webs Markus Grannenfelt / Kimberlite (On hold) Kimberlite digital mines are on hold for the time being. Stay tuned for more info. Kimberlite is also a type of potassic volcanic rock best known for sometimes containing diamonds in it’s rock matrix. It is named after the town of Kimberleyin South Africa, where the discovery of an 83.5-carat (16.7 g) diamond in 1871 spawned a diamond rush, eventually creating the Big Hole.

Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes . Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle .

Question: The volcanic rock Kimberlite is best known for sometimes containing which gemstone?
Answer: Diamond